A detailed model of the molecular mechanisms by which small enveloped viruses infect host cells is a long range goal of our studies. Here, the methods of electron cryo-microscopy (cryoEM), three-dimensional (3D) image reconstruction, and pseudo-atomic modeling will be used to explore the structures, initially, of Sindbis virus (SINV: alphavirus genus - Togaviridae family) and Dengue virus (DENV: flavivirus genus -Flaviviridae family), neither of which has been crystallized in a form suitable for X-ray diffraction studies. However, flavi- and alphaviruses have been intensively studied because of their significant impact on human and animal health and because they have been identified as potential bioterrorist 'weapons'. Flaviviruses comprise a genus of medically important, arthropod-transmitted viruses with more than 70 members that include yellow fever, West Nile, tick-borne encephalitis, and Japanese encephalitis viruses. DENV is one of the most significant human viral pathogens transmitted by mosquitoes. It causes approximately 50 million infections worldwide each year, resulting in around 24,000 deaths. Vaccines have been developed for several flaviviruses, but control of DENV through the use of vaccination has proved to be elusive. Alphaviruses are a group of about 26 arthropod-transmitted viruses, some of which produce an asymptomatic infection in humans. They have been quite useful as gene expression vectors for mammalian and insect cell lines. Others produce fever, rash, and encephalitis in humans and domesticated animals, some with high mortality rates. The E and E1 membrane-anchored glycoproteins (Class II fusion proteins) of flavi- and alphaviruses, respectively, promote fusion of viral and host endosomal membranes in a manner quite distinct from the mechanism carried out by Class I fusion proteins and typified in the well-studied influenza and HIV systems. Determination of the SINV and DENV structures at moderate to high resolution (approximately 5-10 Angstroms) will enable detailed pseudo-atomic models to be constructed and help determine how the viral glycoproteins are organized and participate in low pH triggered fusion. Dedicated efforts to improve methods of specimen preparation, imaging, processing, and visualization are required to markedly improve the resolution in cryoEM structure determinations and will also enable a variety of related structures (e.g. of virions, mutants, intermediates, and recombinant subviral particles) to be studied at similar resolutions. All structural results from cryoEM will be closely guided, correlated and substantiated by coordinated crystallographic and molecular (e.g. mutagenesis) studies conducted by co-PIs Chen, Kuhn, Rossmann, Smith, and Strauss.